1. Field of the Invention
This invention relates to batteries, particularly rechargeable battery packs that have electronics for protecting the battery packs.
2. Description of the Related Art
Rechargeable batteries are used in a variety of electronic devices, including portable computers, portable computer peripherals, personal digital assistants (PDAs), cellular phones, and cameras. Because of the wide variety of uses for rechargeable batteries, a number of different rechargeable battery chemistries have been developed, each having certain advantages and disadvantages. Among the most commonly used battery chemistries are: nickel cadmium (NiCad), nickel-metal hydride (NiMH), lithium ion (Li-ion) and lithium-polymer (Li-polymer).
NiCad batteries have nickel and cadmium electrodes and a potassium hydroxide electrolyte. NiCad batteries are the most common rechargeable batteries, however, NiCad batteries are subject to a number of problems. For example, NiCad batteries have a memory effect, which is a loss of battery capacity caused by recharging the battery before it is fully discharged. Additionally, NiCad batteries are susceptible to over-charging, which causes the battery to develop internal short circuits, thereby causing the battery to run down prematurely which may eventually cause the battery to take no charge at all. Additionally, cadmium is a poisonous heavy metal, and so disposing of NiCad batteries presents additional problems.
NiMH batteries offer higher energy density than NiCad batteries, eliminate many of the disposal problems, and are relatively inexpensive. NiMH batteries have a hydrogen-absorbing alloy anode, a nickel compound cathode, and a potassium hydroxide electrolyte. However, NiMH batteries also have a number of disadvantages. For example, NiMH batteries have a high self-discharge rate, are subject to voltage depression (an effect similar to the memory effect seen in NiCad batteries), and are sensitive to thermal conditions.
In recent years, Li-ion batteries have become the rechargeable battery of choice in devices such as portable computers. The chemistry behind Li-ion batteries involves lithium-plated foil anodes, an organic electrolyte, and lithium compounds within carbon electrodes. Li-ion batteries have very high energy densities, better cycle life than NiMH or NiCad batteries, higher output voltages, and lower self-discharge rates. However, one problem with Li-ion batteries, and potentially other battery chemistries, is the batteries' vulnerability to damage when charged to high capacity (e.g. 85-100% of full capacity) and exposed to high temperatures (e.g. 55.degree. C. and higher).
The complexity of these rechargeable battery chemistries and the complexity of the devices in which such batteries are used requires that rechargeable battery packs be managed by control electronics. Control electronics can be used to monitor and regulate battery pack activity, as well as to interact with other devices such as portable computers or battery chargers. Control electronics include, for example, temperature monitoring circuits, voltage monitoring circuits, current monitoring circuits, controllers, charging circuits, and protection circuits.
Control electronics are particularly important for so-called smart batteries. Smart batteries are rechargeable battery packs equipped with control electronics to provide present capacity information and charging information about the battery to a host device. The control electronics can be embedded in the battery pack, or exist outside the battery pack. Wherever the control electronics are located, the control electronics also monitor the environmental conditions of the battery pack. Smart batteries often maintain information regarding environment, charging characteristics, discharge characteristics, self-discharge characteristics, capacity information, and/or other performance characteristics. Such information can be stored in the battery pack or separate from the battery pack. Smart batteries may also include programmable alarm values for events such as remaining run-time, over-charge, or over-temperature. Based on the battery characteristics, environmental conditions, and measured battery properties (e.g. current and voltage), the control electronics allow the smart battery to accurately determine remaining battery life, power availability and optimal charging conditions. Moreover, this information can be provided to a host device or a smart battery charger.
Rechargeable battery packs often include devices to protect the rechargeable batteries themselves, the battery pack as a whole, or the host device that uses power from the battery pack. For example, many rechargeable battery packs include an internal switch, activated by a controller, to regulate current flow based on the state of the battery pack, for example whether it is in use or not. Fuses are also used to prevent excessive current flow into or out of the battery pack, or to otherwise disable the battery for a host device's protection. For example, if the internal switch of the battery pack has failed, a fuse can be opened by the battery pack's controller so as to prevent unanticipated or uncontrollable current discharge.
Control electronics for rechargeable battery packs often include a controller, typically an integrated circuit microcontroller, or microprocessor. Proper function of the control electronics depends in part on the controller behaving as designed. Consequently, one failure mode for a controller is when the controller is operated under conditions outside of those specified by the controller's manufacturer. An example of such a failure mode is operation of a controller at a temperature below the minimum operating temperature specified by the manufacturer.
One such controller is available from Mitsubishi Electric under the trade designation M37515. The M37515 is a microcontroller typical of those used in rechargeable battery packs. The manufacturer's specification indicates that the minimum operating temperature for the M37515 is -20.degree. C. (-4.degree. F.). Although rechargeable battery packs are not normally used at or below such temperatures, it is not uncommon for battery packs to be exposed to low temperatures during shipping or while in storage. Exposure of the rechargeable battery pack to low temperatures increases the likelihood that the controller will malfunction or fail, which in turn threatens the battery pack. For example, when a controller malfunctions, it may loose the ability to accurately measure current going into or out of the battery pack. If the battery pack is already in a state where its internal switch is turned off, and the controller is erroneously measuring a current flow because the controller is malfunctioning, the controller might conclude that the internal switch has failed. The controller would then cause a protective fuse to be opened because of the perceived problem with the switch. Once the fuse is opened, the battery pack may require service or may be completely unusable. Regardless, the controller's mismeasurement (caused by failure due to low temperature) can lead to unnecessary damage to the rechargeable battery pack.
Accordingly, it is desirable to have a rechargeable battery pack that will protect itself from certain failure modes by preventing its controller from operating at temperatures below a specified level.